Silicon nitride has been investigated for the past several decades as a material which meets high critical engineering requirements for high temperature resistance, strength and other properties. Basically, silicon nitride is formed by nitriding silicon powder in an attempt to form high alpha-phase content silicon nitride material. Thereafter, if additional strength is required, the alpha-phase silicon nitride material is sintered at high temperature in an attempt to form densified beta-phase material. However, due to very long nitriding times, attempts have been made to speed up the nitridation, which has unfortunately brought with it a commensurate problem of the formation of high beta-phase silicon nitride material before sintering. This has caused a problem in that only alpha-phase silicon nitride may be sintered into densified beta-phase silicon nitride material. If the starting material is already in the beta-phase, it will not densify, and therefore the strength will be compromised.
Densification of reaction bonded silicon nitride by sintering is one method of manufacturing a sintered beta-phase silicon nitride. Provided that a high alpha phase content silicon nitride material is produced during the nitridation of silicon, and that sintering or densification aids, which act as liquid forming agents, have been incorporated into the silicon compact either before or after nitridation, it is possible to further densify the reaction bonded silicon compact by heating it to a normal sintering temperature range for silicon nitride.
Densification of silicon nitride occurs by the transformation of the alpha phase of silicon nitride into the beta phase of silicon nitride in the presence of a high temperature liquid phase, accompanied by about a 5-15 percent reduction in volume. The liquid phase promotes the conversion of the densified beta phase silicon nitride during the sintering or densification. It has been found that densification does not generally occur without liquid forming agents. When alpha-phase material is subjected to high temperatures in the absence of liquid forming agents, conversion may be directly to beta-phase material without changes in volume, and consequently no densification.
In the past there have been two major problems associated with the sintering of reaction bonded silicon nitride: (1) the requirement for a high percentage of alpha phase content within the starting material, and (2) the extensive time required for preparation and nitridation of the silicon powder. Typically, in order to manufacture a sinterable reaction bonded silicon nitride, very pure silicon has been ground and mixed dry with densification aids for long periods of time, upwards of 48 hours, and then nitrided for long times, on the order of hundreds of hours to weeks. Total fabrication times of 200 to 400 hours is not uncommon. Previously, impure silicon or attempted fast nitriding rates resulted initially in a high beta phase reaction bonded silicon nitride material which would not densify during sintering. These factors have made it difficult to achieve commercial feasibility on a large scale.
Reaction bonded silicon nitride is commonly prepared by reacting and nitriding the silicon (either as a powder or as a formed article) with nitrogen by exposing the silicon to a nitrogen-containing atmosphere at temperatures of about 1000.degree. C. to about 1420.degree. C. for times sufficient to produce the silicon nitride. It is not uncommon for the nitriding time in prior art methods to be 100-200 hours. It is normal for a small amount of nitriding aid (e.g., iron oxide or nickel oxide) to be initially mixed with the silicon powder to enhance the nitridation of the silicon during the nitriding step. This nitridation has traditionally been slow due to long holding times at particular temperatures to avoid overheating and also to avoid the formation of beta-phase material.
It is widely known that if common sintering or densification aids for silicon nitride (e.g., magnesium oxide, yttrium oxide, aluminum oxide, rare earth oxides, etc.) are mixed in initially with the silicon powder, the reaction bonded silicon nitride article, provided it has a high alpha phase content, can be further heated immediately after nitriding to the higher sintering temperature and will thereby be sintered to increase its density and have improved mechanical properties as compared to the original reaction bonded silicon nitride article. The article can be hot pressed, hot isostatically pressed, pressure assisted sintered, or pressureless sintered, and may be covered with a cover powder during the sintering process to prevent any degradation. Boron nitride or silicon nitride, or mixtures thereof are commonly employed as a cover powder over the article during sintering to minimize the decomposition of the reaction bonded silicon nitride article.
During sintering, the alpha phase of the silicon nitride material is converted to the beta phase of silicon nitride. Therefore, high levels of alpha phase silicon nitride needs to be present in the pre-sintered reaction bonded silicon nitride to obtain the desired sintering response. In the past, it has been found that some methods produce a high beta phase material too early in the process to be useful.
U.S. Pat. No. 4,285,895 to Mangels et al. teaches that sintered reaction bonded silicon nitride articles can be made by incorporation of a densification aid into the reaction bonded silicon nitride article, surrounding the article with a packing powder of silicon nitride and densification aid and subjecting the article and powder mixture to a temperature above 1700.degree. C. with a nitrogen atmosphere of sufficient pressure to prevent volatilization of the silicon nitride for a time sufficient to permit sintering of the reaction bonded silicon nitride articles.
Several methods for introducing the densification aid into the reaction bonded silicon nitride article are disclosed in the above referenced Mangels et al. patent. These include (1) the impregnation of the densification aid into the reaction bonded silicon nitride article; (2) incorporation of the densification aid into the cover powder and then relying upon diffusion of that densification aid into the article at the sintering temperature; and (3) incorporation of the densification aid into the silicon powder mix prior to nitriding. The densification aids cited are magnesium oxide, yttrium oxide, cerium oxide, and zirconium oxide. The Mangels et al. patent also teaches that the nitrogen pressure at the sintering temperature may be in the range of 250 to 1500 psi.
U.S. Pat. No. 4,351,787 to Martinengo et al. teaches that sintered silicon nitride articles can be prepared by forming a silicon powder mixture containing one or more sintering additives into a compact, the additives being present in the powder in an amount such as to ensure an additive content of from 0.5 to 20 wt % in the silicon nitride compact; heating the compact under a nitrogen gas blanket at a temperature not exceeding 1500.degree. C. to convert the silicon into reaction bonded silicon nitride; and sintering the reaction bonded silicon nitride compact by heating in a nitrogen gas atmosphere at a temperature of at least 1500.degree. C. Furthermore, it is taught that the silicon powder size is from 0.1 to 44 microns in size and of high purity or containing only very small amounts of nitriding catalysts. The Martinengo et al. patent teaches that any conventional sintering additive may be used. Best results are said to be achieved by using MgO, and especially in combination with Y.sub.2 O.sub.3. Other preferred additives mentioned in the patent are MgO, Y.sub.2 O.sub.3, CeO.sub.2, ZrO.sub.2, BeO, Mg.sub.3 N.sub.2, and AlN. Other examples of additives are given as Mg.sub.2 Si, MgAl.sub.2 O.sub.4, and rare earth additions such as La.sub.2 O.sub.3. Also iron can be used with advantage, usually in mixture with conventional additives such as MgO, Y.sub.2 O.sub.3, and CeO.sub.2.
As a final example of sintered reaction bonded silicon nitride practice, reference is made to U.S. Pat. No. 4,443,394 to Ezis which teaches a method for making a fully densified silicon nitride body. The basic principle taught is that silicon nitride will not sinter by itself, but requires a liquid phase at the sintering temperature. Ezis found that, by having an yttrium oxynitride and alumino-silicate liquid phase present at sintering temperatures of 1650.degree.-1750.degree. C., the need for an over pressure of nitrogen and cover or packing powder during sintering could be eliminated in order to densify the silicon nitride.
The Ezis patent teaches that, by (1) forming a nitridable mixture of: silicon powder, SiO.sub.2 (carried with the Si metal), Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 ; (2) nitriding the mixture to form a reaction bonded silicon nitride, with consequent formation of a Y.sub.10 Si.sub.6 O.sub.24 N.sub.2 phase, and an alumino-silicate which resides on the silicon nitride grains; and then (3) sintering in the 1650.degree. to 1750.degree. C. temperature range for 5-12 hours, a substantially fully densified silicon nitride is produced which exhibits a 4-point bending strength of 100,000 psi at room temperature.
The Ezis patent further teaches the need for a long ball milling time of 48 hours, preferably dry, a nitridation cycle time of 200 hours, and sintering times of 5-12 hours. Total processing time including the milling can be estimated from the preferred embodiment as approximately 260 hours.
Many of the densification aids mentioned above or others used in the past are relatively expensive, are not always readily available, and require relatively high sintering temperatures for effectiveness.
It is, therefore, a primary object of the present invention to provide improved processes for preparing an alpha-phase silicon nitride material which can later be sintered to form a substantially densified beta-phase material which is more commercially viable than prior art methods, uses materials which are readily available, and is processable in substantially less time than typical prior art methods.